1. Field of the Invention
This invention relates to gaseous discharge lamps which ignite at voltages that are much higher than their operating voltages and, in particular, to the igniting of such lamps.
2. Description of Related Art
Common characteristics of a gaseous discharge lamp are its negative resistance and high igniting voltage. A circuit arrangement for powering such a lamp typically includes a current limiting means, such as a ballast, to compensate for the negative resistance, and often includes igniter circuitry for generating high-voltage pulses to ignite the lamps. Such igniter circuitry commonly includes a voltage-sensitive switch (e.g. a sidac) for effecting the continual production of the high-voltage pulses until the lamp ignites. Upon ignition, the voltage across the lamp decreases from a higher open-circuit voltage (OCV) to a lower voltage, which causes the switch to change to a non-conducting state and to effect termination of pulse production. One example of such a ballast is described in U.S. Pat. No. 5,319,286.
In some situations, the igniter circuitry may be overstressed to the point where the voltage-sensitive switch fails. This is particularly a problem with igniter circuitry which repeatedly applies such high-voltage pulses to a lamp which cannot be stably ignited.
It is an object of the invention to provide circuitry for igniting a gaseous discharge lamp which reduces stress on the voltage-sensitive switch during generation of the igniting pulses.
A common circuit arrangement for igniting a gaseous discharge lamp includes an inductive pulse generating circuit, including a voltage sensitive switch, and a timing circuit including a timing capacitor for determining how frequently the pulses are produced. It has been found that such circuit arrangements may produce AC currents through the switch which both increase stress on the switch and may adversely affect the operation of the timing circuit. In the case of a lamp which cannot be stably lighted, or one which requires many igniting pulses to be brought into a stable ignition state, such AC currents may affect rapid charging and/or discharging of the capacitor such that the timing circuit permits repeated pulse generation at a rate higher than can be tolerated by the switch.
In accordance with the invention, a circuit arrangement for igniting a gaseous discharge lamp comprises a timing circuit including a timing capacitor for limiting the rate of pulse production and an inductive pulse generating circuit including a unidirectional voltage-sensitive switch that is electrically connected in series with the capacitor. This arrangement unidirectionally limits the series current through the switch and the capacitor during each pulse.